As starch is a vegetable carbohydrate, there have been attempts to use it as a "natural" plastic material in a wide variety of areas, employing known plastics processing methods. However, owing to their granular structure, native starches first have to be destructured before they can become thermoplastically processible.
To impart the good properties known and desired of plastics materials to an opaque basic starch molding composition of this type, it is necessary to process it further. When this is done, it often loses its biodegradability.
EP 344 118 A2 relates to a polymer blend material consisting of a melt of at least one water-containing destructured hydrophilic polymer and at least one synthetic, substantially water-insoluble thermoplastic polymer. The dimensional stability of the material is improved by the addition of the water-insoluble polymer; this is demonstrated by examples of blends based on gelatins, as well as cellulose derivatives, and polyethylene, polystyrene, polyvinylether, polyoxymethylene, and ethylene acrylic acid copolymers. The patent's reference to possible biodegradability, namely the loss of this additional stability after several days, is considered to be a disadvantage for the products shaped from the blends; no comment is made as to the biodegradability of the blend.
EP-OS 327 505 A2 describes a melt-mixed polymer blend consisting of destructurized but chemically unmodified starch and at least one water-insoluble synthetic thermoplastic polymer. Similarly, polymer blend compositions consisting of chemically unmodified destructurized starch and a functionalized polymer are known from the laid open and EP Applications 409 789 A2, 409 788 A2, 409 783 A2, 409 782 A2, 409 781 A1, 408 503 A2, 408 502 A2, 408 501 A2, 407 350 A2, 404 728 A2, 404 723 A2, and 404 727 A2. These compositions may each additionally contain a further water-insoluble thermoplastic polymer. In that case, the functionalized polymer acts as a compatibilizer between the chemically unmodified starch and the additional third thermoplastic polymer.
German Patent Application DE 41 19 915.4 describes starch polymer mixtures consisting of a starch molding composition and selected linear polymers. Their specific disadvantage, as well as that of all starch materials disclosed in the above-mentioned specifications, is their typical yellow tinge as well as their non-transparency. Opaque material can be used only to a limited extent, in particular in the packaging field, as transparency of the plastic materials is usually required.